In general, an OLED is an emissive device and has been highlighted as the next generation display because it not only has a wide viewing angle and a good contrast but also a fast response time.
The OLED comprises an anode, a cathode and an emission layer (hereinafter, referred to as EML) interposed between the anode and the cathode. When a voltage is applied between the anode and the cathode, holes and electrons are injected into the EML, and then combined in the EML to create exitons, which decay radiatively. This radiation is called electroluminescence (EL)
The luminance property of the OLED may be determined in accordance with electroluminescent property of an organic material used for forming the EML. U.S. Pat. No. 4,769,292 discloses an OLED having a luminescent zone of less than 1 μmin thickness comprised of an organic host material and a light-emitting dopant. In this case, the light-emitting dopant may be a material that receives energy from the host material to create excitons, which are subject to transition from the excited state to the ground state to thereby emit light. The light-emitting dopant may act to adjust the color of the emitted light and improve the luminous efficiency of the OLED.
As such, when the EML is comprised of the host material and the dopant, the concentration of the dopant affects the driving voltage and the luminous efficiency of the OLED. The driving voltage may be reduced by increasing the concentration of the dopant. However, in this case, a concentration quenching phenomena may occur to thereby reduce the luminous efficiency. Thus, the dopant concentration needs to be adjusted to allow the driving voltage to be reduced and the luminous efficiency to be increased.
The excitons are created in the EML and remain in the excited state for a predetermined period of time and subject to transition to the ground state. The time taken for this procedure is referred to as the lifetime of the excitons. The excitons may be diffused into an interface between the EML and the anode or an interface between the EML and the cathode during the lifetime. Therefore, the excitons need to be confined within the EML to enhance the luminous efficiency of the OLED. This may be more necessary when using a phosphorus dopant having a long diffusion distance and excitons with a long lifetime.
U.S. Pat. No. 6,097,147 discloses a technique that can enhance the luminous efficiency by interposing a blocking layer between the EML and the cathode to confine the excitons within the EML. However, forming the blocking layer as a common layer on the red (R), the green (G) and the blue (B) EMLs in the process of fabricating a full color OLED, makes it difficult to adjust each of the luminous efficiencies of R, G, and B pixels.